WLAN having load balancing by access point admission/termination

ABSTRACT

A network includes access points that admit/terminate mobile station associations based upon the loading level of the access point and/or whether a mobile station can associate with a further access point. Mobile stations transmit information indicative of the access points to which they can associate. The access points determine whether to admit/terminate a mobile station association based upon access point loading.

CROSS REFERENCE TO RELATED APPLICATIONS

The present invention claims the benefit of U.S. Provisional PatentApplication No. 60/332,956, filed on Nov. 19, 2001, which isincorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not Applicable.

FIELD OF THE INVENTION

The present invention relates generally to communication networks and,more particularly, to wireless communication networks.

BACKGROUND OF THE INVENTION

Wireless communication networks for providing links to mobile stationsare well known in the art. In one type of wireless network, a series ofaccess points provide wireless connections to various mobile users. Forexample, a building can include access points located at strategiclocations to serve mobile users as they move throughout the building.The mobile users migrate from access point to access point based uponthe strength of beacon signals from the various access points. That is,the mobile stations use the strength of the beacon signals to select thebest access point at a given point in time.

With changes in the channel environment and number of users in aWireless Local Area Network (WLAN) system, different access pointsexperience different loading. That is, the number of users served byeach of the access points varies over time. Those access points thatserve a relatively high number of stations (hot spots) can becomeoverloaded and experience reduced performance. For example, an accesspoint can become overloaded during a meeting in a conference roomproximate the access point when the attendees attempt to connect theirlaptops to the corporate intranet.

It would, therefore, be desirable to adjust the loading of networkaccess points to reduce network congestion.

SUMMARY OF THE INVENTION

The present invention provides a wireless network having access pointswith mobile station association admission/termination control for accesspoint load balancing in accordance with the present invention. With thisarrangement, overall network performance is enhanced by more efficientaccess point loading. While the invention is primarily shown anddescribed in conjunction with wireless access points in a 802.11network, it is understood that the invention is applicable to wirelessnetworks in general in which it is desirable to distribute loading.

In one aspect of the invention, a wireless network includes a series ofaccess points to which mobile stations can associate. In the case wherean access point becomes relatively heavily loaded, or overloaded, theaccess point can decline association requests from additional mobilestations. In addition, an access point may terminate an existingassociation under certain conditions, such as where an associated mobilestation has the option to associate with a further access point.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood from the following detaileddescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a schematic depiction of a wireless network having accesspoints that can decline/terminate mobile station associations inaccordance with the present invention;

FIG. 2 is a schematic block diagram of an exemplary access point thatcan decline/terminate mobile station associations in accordance with thepresent invention;

FIG. 3 is a pictorial representation of an exemplary capability fieldthat can be contained in a message exchanged by an access point and amobile station in accordance with the present invention;

FIG. 4A is a schematic depiction of a mobile station for associatingwith an access point that can decline/terminate associations inaccordance with the present invention;

FIG. 4B is a schematic depiction of an access point that candecline/terminate associations in accordance with the present invention;and

FIG. 5 is a schematic depiction of a network having an access pointcontroller for controlling loading of network access points inaccordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an exemplary wireless network 100 having access points thatperform load balancing by access point admission control of mobilestations and/or termination of mobile station associations. The network100 includes a series of access points AP_(a-N) having associatedcoverage areas CA_(a-P) serving various mobile stations MS_(a-M). Ingeneral, each access point Ap_(a-N) evaluates its loading level todetermine whether additional mobile station associations are feasible,e.g., would not result in (or make worse) an overload condition for theaccess point. In addition, an access point can terminate a mobilestation association under certain conditions to distribute access pointloading. For example, a mobile station may transmit an indication to anoverloaded access point that there are other access points to which themobile station can associate itself. The overloaded access point mayterminate the association and the mobile station can attempt toassociate with another access point having a lower traffic loadinglevel.

For example, a relatively highly loaded access point, here shown asAP_(a), can determine that it should not accept additional associations,such as from MS_(c). Or this access point AP_(a) may terminate anassociation with the mobile station MS_(c). In one embodiment, theaccess point terminates an association with the mobile station havingthe greatest number of access point options. This mobile station MS_(c)then looks for another access point, e.g., AP_(b), to which anassociation request can be made, as described in detail below.

In an exemplary embodiment, after an association is terminated, themobile station “remembers” which access point terminated the associationand does not attempt to associate with that access point until apredetermined event occurs, e.g., a time interval expires, etc. Thisprevents the mobile station from bouncing around between access points.

In one embodiment, the access points AP receive information from themobile stations MS indicating how many access points each mobile stationcan “hear.” As used herein, a mobile station can “hear” an access pointif the associated beacon signal is received by the mobile station at orabove a predetermined power level. However, further techniques, such aserror rates, for evaluating a link between an access point and a mobilestation will be readily apparent to one of ordinary skill in the art.

It is understood that a determination as to whether an access pointshould accept an additional association or terminate an existingassociation can be based upon a wide variety of factors, includingnumber of associated mobile stations, link bandwidth, expected trafficlevels, traffic intensity, packet error rate, packet delay at the accesspoint, and the measurement of the received signal power from a subset ofstations. It is understood that these factors can be considered in arange of techniques, such as weighting selected factors.

It is further understood that the term “mobile station,” as used herein,should be construed broadly to include various wireless devices, such aslaptops, Personal Digital Assistants (PDAs), mobile phones, and thelike. Similarly, the term “access point” should be broadly construed toinclude transmitters/receivers in general that can provide a radio linkwith a mobile station.

Before describing further details of the present invention, some basicconcepts are now described. In conventional wireless networks havingmobile stations served by various access points, such as in a 802.11network, there is a standard procedure by which mobile stationsassociate themselves with an access point. The IEEE 802.11 standard isdefined in International Standard ISO/IEC 8802-111, “InformationTechnology-Telecommunications and Information Exchange Area Networks,”1999 Edition, which is hereby incorporated by reference in its entirety.Before a mobile station associates with an access point, it obtainsinformation of nearby access points by scanning the frequency channelsfor their beacons. The access points typically send out beacon flamesperiodically.

In traditional WLANs, such as 802.11 networks, beacon powers of accesspoints are kept at a fixed level. The mobile station simply chooses theaccess point with the best signal strength for association. However, itis possible that one access point may be already overloaded, although ithas the strongest signal strength to the mobile station.

While the following descriptions are applicable to 802.11 WLANs, it isunderstood that the invention applies to wireless networks in generalusing similar formats and mechanisms. Table 1 below shows the beaconframe body of a management frame of subtype Beacon with each of thelisted components specified in the 802.11 standard.

TABLE 1 Beacon frame body Order Information Notes 1 TimestampTime/day/etc information 2 Beacon Time between beacon intervals interval3 Capability Resource parameters, polling parameters, etc. information 4SSID Service Set ID (Wireless Network Name) 5 Supported Data Ratessupported by the system/device rates 6 FH Parameter The FH Parameter Setinformation element is Set present within Beacon frames generated bySTAs (Mobile STAtions) using frequency-hopping (FH) PHYs (PHYsical layermodulations) 7 DS Parameter The DS Parameter Set information element isSet present within Beacon frames generated by STAs using direct sequence(DS) PHYs 8 CF Parameter The CF (Contention Free) Parameter Set Setinformation element is only present within Beacon frames generated byAPs (Access Points) supporting a PCF (Point Coordination Function) 9IBSS The IBSS (Independent Basic Service Set) Parameter Set ParameterSet information element is only present within Beacon frames generatedby STAs in an IBSS 10 TIM The TIM (Traffic Information Map) informationelement is only present within Beacon frames generated by APs

Typically, the mobile station can operate in either Passive Scanningmode or Active Scanning mode. In Passive Scanning mode, the mobilestation listens to each channel scanned for no longer than a maximumduration defined by the ChannelTime parameter. The Active Scanning modeinvolves the generation of a Probe Request frame by the mobile stations,which is shown in Table 2 below, and the subsequent processing of areceived Probe Response frame, which is shown in Table 3, by the accesspoint.

TABLE 2 Probe Request frame body Order Information 1 SSID 2 Supportedrates

TABLE 3 Probe Response frame body Or- der Information Notes 1 TimestampTime/day/etc. information 2 Beacon Time between beacon intervalsinterval 3 Capability Resource parameters, polling parameters, etc.information 4 SSID Service Set ID (Wireless Network Name) 5 SupportedData Rates supported by the system/device rates 6 FH Parameter The FHParameter Set information element is Set present within Beacon framesgenerated by STAs using frequency-hopping PHYs 7 DS Parameter The DSParameter Set information element is Set present within Beacon framesgenerated by STAs using direct sequency PHYs 8 CF Parameter The CFParameter Set information element is Set only present within Beaconframes generated by APs supporting a PCF 9 IBSS Parameter The IBSSParameter Set information element is Set only present within Beaconframes generated by STAs in an IBSS

After scanning the access point beacons, the mobile station adopts theBSSID (Basic Service Set ID: Access Point ID) and channelsynchronization information in a Beacon (passive) or Probe Response(active) coming from the access point with the best signal strength. AnAssociation/Reassociation Request is then issued by the mobile stationas it attempts to associate with the selected access point. The accesspoint then responds with an Association Response. The correspondingAssociation Request and Association Response frame formats are shownbelow in Table 4 and Table 5, respectively. It is understood thatfurther actions, such as authentication, take place before or after theassociation phase.

TABLE 4 Association/Reassociation Request frame body Order Information 1Capability information 2 Listen interval 3 SSID 4 Supported rates

TABLE 5 Association Response frame body Order Information 1 Capabilityinformation 2 Status code 3 Association ID (AID) 4 Supported rates

In accordance with the present invention, an access point canadmit/terminate mobile station associations based upon the loading levelof the access point. The mobile station transmits information relatingthe number of access points that are available for association and theaccess points keep track of loading levels. Based upon mobile stationoptions for association and access point loading, each access pointaccepts or rejects new association requests and selectively terminatesexisting associations to optimize overall access point load levels.

FIG. 2 shows an exemplary access point 200 having a processor 202 forcontrolling the overall operation of the access point and a memory 204.The access point 200 further includes an association control module 206for handling mobile station association admission/termination, asdescribed more fully below. The access point 200 can further includeconventional components including a wireless interface 250 including oneor more RF transceivers, a network interface 252 for interacting with awired network, and an I/O interface 254 for communicating with variouscomponents, such as peripheral equipment.

Referring again to FIG. 1, some mobile stations, such as MSc, willreport that they can hear not only the currently associated access pointAP_(a), but also one or more additional access points AP_(b). Ingeneral, mobile stations can “hear” an access point if the beacon signalfrom the access point has a received signal strength at or above apredetermined threshold. Other mobile stations, e.g., MS_(e), willreport that they can only hear their current access point AP_(a).

Due to a termination of an existing association between an access point,e.g., AP_(a), and a mobile station, e.g., MS_(c), the mobile stationMS_(c) can attempt to associate itself with a further access point,e.g., AP_(b). This in turn reduces the load and traffic intensity overthe current access point AP_(a).

Referring now to FIG. 1 in conjunction with FIG. 2, the signalinformation transmitted by the mobile station MS_(a-N) can be stored inthe access point memory 204, such as in a database, which can store, forexample, a list 204 a of mobile stations and the access points that areavailable to each mobile station. The memory can also store a receivedsignal strength 204 b for each mobile station/access point. For singleaccess point mobile stations, e.g., MS_(e), the current access pointAP_(a) can measure the received signal strength.

In an exemplary embodiment, frequency option information can be conveyedin the Association/Reassociation request frame, which is shown above inTables 4 and 5. These two management frames contain the same CapabilityInformation field, which is used to indicate requested or advertisedcapabilities. In an illustrative embodiment, the length of theCapability Information field is two octets.

FIG. 3 shows an exemplary Capability Information field 300 having subfields of ESS 300 a, IBSS 300 b, CFPollable 302 c, CFPoll Request 302 d,and Privacy 302 e, together using five of the sixteen total bits. In anexemplary embodiment, three bits form a further subfield APFC 300 f usedto indicate access points that a given mobile station can associate within frequency channels other than the frequency for the currentlycommunicating access point. The remaining bits 300 g of the CapabilityInformation field 300 are reserved.

In another embodiment the capability information field uses one bit toindicate whether there is an alternative access point with sufficientsignal strength available to the mobile station and uses additional bits(e.g., two to three bits) to indicate the rough signal quality of thealternative access point(s). This information can be used later by thecurrent access point to assess if the particular mobile is able to moveto the alternative access point.

FIG. 4A show an exemplary mobile station 400 that can communicate accesspoint signal information to network access points in accordance with thepresent invention. The mobile station 400 includes an associationfunction 402 that determines the number of access points which themobile station can associate with as included in the CapabilityInformation field in the Association/Reassociation request frame, thusconveying the information to the current access point. In oneembodiment, the association function 402 scans the frequency channels todetermine those access points for which the received beacon signal isgreater than a predetermined threshold, for example. For each accesspoint beacon signal that is greater than the threshold, the mobilestation can consider those access points as viable options in the eventof an association request denial or association termination. Later, thisinformation can be kept updated by a reassociation function 404 viareassociation requests by the mobile when there is a change, e.g.,another access point is no longer available for the mobile station. Thereassociation function 404 can indicate the number of access points thatthe mobile station can potentially associate with as identified in theCapability Information field.

As shown in FIG. 4B, an exemplary access point 500 can include a firstlocal function 502 to assess the medium load condition, e.g., determinethe number of mobile stations associated with the access point, theirbandwidth usage, transmission queue length, number of error packetscompared to the total traffic, etc. The access point 500 can furtherinclude a second local function 504 to assess the likelihood that acurrently associated mobile station can associate with a differentaccess point. More particularly, the access point 500 evaluates whetherthe signal strength of a further access point to the mobile station issufficient to enable migration to the further access point based uponinformation in the mobile station database 204 of FIG. 2, for example.The access point 500 can further include an association control module506 for determining whether to accept association requests and when toterminate current associations, as described above.

In an exemplary embodiment, an access point transmits a disassociationframe to a mobile station in the event that the access point hasdetermined that an association should be terminated to enhance theoverall access point loading. The disassociation frame body can containa reason code associated with the termination. An exemplary list ofreason codes is set forth below in Table 6.

TABLE 6 Reason Codes REASON CODE MEANING 0 Reserved 1 Unspecified 2Previous authentication invalid 3 Deauthenticated because sendingstation is leaving (or has left) IBSS or ESS 4 Disassociated due toinactivity 5 Disassociated because AP is unable to handle all currentlyassociated stations 6 Class 2 frame received from non-authenticatedstation 7 Class 3 frame received from non-associated station 8Disassociated because sending station is leaving (or has left) BSS 9Station requesting (re)association with responding station 10 and upReserved

Upon receiving the disassociation frame, the mobile station stores thereason code and the transmitting access point. The mobile station,through the association module 402 of FIG. 4A, for example, thenattempts to associate with a further access point. However, theassociation module 402 will not subsequently attempt to associate withthe terminating access point. By remembering that a particular accesspoint has terminated an association, the mobile station avoids sendingassociation requests to that access point. The mobile station can treatan access point that has denied an association request in a similarmanner.

It is understood that the mobile station can attempt to associate with aterminating/denying access point after a predetermined event, such as anexpired time duration. A variety of other such events will be readilyapparent to one of ordinary skill in the art.

In an alternative embodiment shown in FIG. 5, access pointassociation/termination decisions are controlled by a central accesspoint controller 600 coupled to the access points 602 a-N in thenetwork. The controller 600 receives association and loading informationfrom the access points 602 and evaluates the overall impact of requestedmobile station association requests and mobile station associationterminations for each access point to achieve optimal load balancing inthe overall network.

One skilled in the art will appreciate further features and advantagesof the invention based on the above-described embodiments. Accordingly,the invention is not to be limited by what has been particularly shownand described, except as indicated by the appended claims. Allpublications and references cited herein are expressly incorporatedherein by reference in their entirety.

1. A method of controlling access point loading in a wireless network,comprising: receiving information transmitted by a mobile station to afirst one of a plurality of access points, the information including anindication of one or more access points in the plurality of accesspoints to which the mobile station can attempt to associate; anddetermining whether to admit/terminate an association of the mobilestation to the first one of the plurality of access points based upon aloading level of the first one of the plurality of access points.
 2. Themethod according to claim 1, further including determining whether themobile station can establish an association with a second one of theplurality of access points.
 3. The method according to claim 2, furtherincluding terminating an association between the mobile station and thefirst one of the plurality of access points and establishing anassociation between the mobile station and the second one of theplurality of access points.
 4. The method according to claim 3, furtherincluding storing an identification of the first one of the plurality ofaccess points.
 5. The method according to claim 4, further including notrequesting an association with the first one of the plurality of accesspoints by the mobile station based upon the association termination. 6.The method according to claim 1, further including not requesting anassociation with the first one of the plurality of access points by themobile station after being denied an association.
 7. The methodaccording to claim 1, further including receiving the information fromthe mobile station an association request frame.
 8. The method accordingto claim 7, further including receiving the information from the mobilestation in a capability information subfield within the associationrequest frame.
 9. The method according to claim 1, further includingstoring the information in a database.
 10. The method according to claim1, further including determining the loading level of the first one ofthe plurality of access points by at least determining a number ofcurrently associated mobile stations.
 11. The method according to claim1, further including determining a loading level of the first one of theplurality of access points by determining one or more of a number ofcurrently associated mobile stations, link bandwidth, traffic levels,and a measurement of received signal power from a subset of mobilestations.
 12. An access point, comprising: a transceiver for providingcommunication with a mobile station over a radio link; a memory forstoring information including for one or more mobile stations how manyaccess points each of the one or more mobile stations can associatewith; and an association module for determining whether toaccept/terminate an association with a mobile station based upon aloading level of the access point and the stored information for the oneor more mobile stations.
 13. A mobile station, comprising: a transceiverfor providing communication with an access point over a radio linkincluding transmitting a message containing information relating toaccess points with which the mobile station can associate.
 14. Themobile station according to claim 13, further including a memory forstoring an identification of an access point that has denied/terminatedassociation with the mobile station.